This disclosure relates generally to magnetic sensors, and more particularly to the low cost integration of micron-scale permanent magnets with magnetic sensors for generating a large, relatively uniform perpendicular bias field that may be used to maximize the sensitivity of semiconductor magnetoresistance sensors.
Semiconductor magnetoresistance sensors are a promising class of solid-state magnetic sensors. These sensors consist of a substrate of patterned, high mobility semiconducting films. Some non-limiting examples of patterned, high mobility semiconducting films include indium antimonide (InSb), indium arsenide (InAs), gallium antimonide (GaSb), gallium arsenide (GaAs) and grapheme. The application of a perpendicular magnetic field to the substrate of a patterned, high mobility semiconducting film deflects the current in the substrate of the patterned, high mobility semiconducting film, resulting in an increased path length and hence an increased resistance. By optimizing the geometry of the semiconductor magnetoresistance sensor, the sensitivity can be maximized.
While magnetoresistance sensors have been developed for some time, they have not had broad commercial applicability, due in part to the need to apply a large perpendicular magnetic bias field (approximately 0.1 to 0.2 Tesla) to achieve high sensitivity. For certain applications such as clearance sensors for automotive applications, macroscopic permanent magnets are either already present or can be easily integrated into a desired location. Thus, magnetoresistance sensors have been intensively investigated for automotive applications.
However, a much larger range of magnetic sensor applications require that the entire assembly (sensor and magnet) must be compact. Examples include surface mount semiconductor packages and electromagnetic tracking devices for medical instruments, such as needles, catheters and guidewires, etc.
Macroscopic permanent magnets are typically fabricated by pressure sintering permanent magnet powder (e.g., neodymium iron boron (NdFeB)) into a desired form. While these magnets are capable of achieving very large magnetic fields on their faces (approximately 0.5 Tesla) they cannot be shrunk down to less than approximately 1 mm3 volumes needed for space constrained applications. In addition, as each magnet is fabricated separately, precise placement and bonding of the magnet within the magnetoresistance sensor is very difficult.
Alternatively, perpendicular magnetic bias fields can be generated using magnetic thin films with perpendicular anisotropy. Examples include iron gadolinium terbium (FeGdTb) alloys and a cobolt platinum (CoPt) multilayer. Unfortunately, however, to generate a large (approximately 0.1 to 0.2 Tesla) uniform magnetic field over the front face requires that the thickness of the film be approximately as large as the base (dependent upon the detailed magnetic properties of the material). Thus, a magnetic sensor with an active area of approximately 0.25 mm×0.25 mm would require a permanent magnet material that is at least approximately 0.15 mm thick (dependent upon the detailed magnetic properties of the material). At this film thickness, traditional thin film process techniques such as sputtering, evaporation or chemical vapor deposition are not feasible. While electroplating has been used to create magnetic films of thicknesses up to approximately 30 μm, the magnetic properties are too poor for magnetic field values needed for magnetoresistance sensors.
Therefore, there is a need for low cost integration of micron-scale permanent magnets within magnetic sensors for generating a large, relatively uniform perpendicular magnetic bias field that may be used to maximize the sensitivity of semiconductor magnetoresistance sensors.